1. Field of the Invention
The present invention relates to a process for coating or hardfacing a metal part by the deposition of a metal using the plasma transferred arc method. It more particularly applies to the coating or hardfacing of parts such as the valves of internal combustion engines.
2. Discussion of Background
Coating/hardfacing procedures consist of producing relatively thick deposits of a material on a substrate in order to give the latter the qualities inherent in the deposited material.
According to this procedure, between the substrate and the deposited material a metallurgical bond is produced by welding, which makes it necessary to heat the substrate in order to melt its surface during the supply of the coating material in the melted state. Thus, in the substrate appears a thermally influenced zone in the vicinity of the deposit and a dilution of the substrate in the deposited material.
Hardfacing operations of this type can be carried out with conventional welding equipment such as oxyacetylene torches, as well as equipment using an electric arc such as tungsten inert gas (TIG torches), metal inert gas (MIG torches) and plasma transferred arcs (PTA). These welding equipments are continuously or discontinuously supplied with filler material, in the form of powders, rods or wires.
When use is made of the plasma transferred arc method, the transferred arc constitutes the heating element of the recharging material and the surface of the part forming the substrate. The part to be treated is raised to a positive potential compared with the cathode of the torch and the plasma jet is then entirely traversed by electric current between the torch and the substrate, transmitting to the latter the energy necessary for heating and localized melting of the zone to be coated or hardfaced. The filler material is melted on the surface of the part in order to form a liquid bath, which is continuously renewed during the displacement of the part beneath the torch. The surface melting of the substrate permits a metallurgical bond identical to that encountered in welding processes.
With the presently used, conventional equipment, whose nozzles have a diameter of 2.5 to 4.5 mm, the material is deposited in the form of a weld bead by moving the part relative to the torch at a relative torch-part speed between 0.08 and 1.10 m/min and using average current densities of 5 to 12 amp/mm.sup.2 at the nozzles.
With such energy levels, in the case of small parts such as valves, it is necessary to cool the part during deposition in order to avoid a modification of its properties due to the heating. This cooling is generally carried out by placing the part on a support internally cooled by a circulation of water.
By means of these methods, it is generally possible to obtain weld beads having a rounded profile, i.e. having a deposited material increased thickness and relatively high average dilution rates of e.g. approximately 8 to 12%.